Cognitive development has often been depicted as if there were a 1:1 correspondence between age and strategy use. Young children would use one strategy, somewhat older children a different strategy, yet older children a third strategy, and so on. Recent research, however, has shown that in domains as diverse as arithmetic, serial recall, causal and spatial reasoning, referential communication, and judgments of plausibility, children of a given age often use multiple strategies. This is true within as well as between individuals, and even within individuals solving the same problem on two consecutive days. The use of multiple strategies enhances children's effectiveness in adapting to differences among problems and to varying situational demands. In the proposed project, the investigators hope to extend strategy choice research in a number of ways. They intend to: (1) demonstrate that the capacity to choose adaptively among strategies is a basic characteristic of human beings, by showing that even one-year-olds can choose strategies adaptively; (2) provide evidence that on problems where no single strategy is invariably most effective, that strategy diversity increases, rather than decreasing, with age and experience; (3) investigate the important but understudied process of quantitative estimation through use of the strategy choice approach; (4) examine discovery and generalization of new strategies, in particular serial recall and number conservation strategies, thorough microgenetic analyses of change over a number of sessions; and (5) formulate computer simulations of strategy choice and strategy discovery in the contexts of serial recall and arithmetic. By proceeding in these directions, they hope to broaden the scope of strategy choice research, to address several fundamental issues in the area, and to advance understanding of the mechanisms through which children choose among existing strategies and discover new ones.